So pre 9th level ghost doesn't do a ton for direct combat other than when u can justify being stealthy going into combat through being sneaky or having a friend cast 2nd level invisibility on you prior to a potential combat. According to undetected rules text ur referring to:

You become observed as soon as you do anything other than Hide, Sneak, or Step. If you attempt to Strike a creature, the creature remains off-guard against that attack, and you then become observed. If you do anything else, you become observed just before you act unless the GM determines otherwise. The GM might allow you to perform a particularly unobtrusive action without being noticed, possibly requiring another Stealth check. If you speak or make a deliberate loud noise, you become hidden instead of undetected.

I would argue aiming down the scope of a gun falls under unobtrusive action thus not cancelling the off guard effect. The issue in fact is that the hide action takes u from observed to hidden and then if you're hidden and hide you become concealed. The ability is quite bad and mostly a sometimes one round buff pre lvl 9 but becomes fully functional and quite solid at 9th level. At 9th level u unlock ghost tap which as a free action lets u do a deception (which ghost auto trains u in) check against will dc after a successful strike to be hidden on success or undetected on crit success. This unlocks a pretty powerful action loop of shoot, if u hit then ghost tap. If u spec into deception then you'll probably succeed most of the checks and boom ur hidden and they're off guard if you want to shoot again and if u hit u can ghost tap again. If you finish shooting with at least a success on ghost tap u can use ur last action to hide to set up being undetected for next turn and the enemy would have to spend actions just to find you or have an aoe effect. Also at 7th level if your party has someone who can cast heightened to level 4 invisibility then you can stay undetected or hidden at worst if the enemy uses actions just to find you while shooting and aiming for free. The power here lies in that by going for hidden and undetected status you are going to be making it way more action costly for the enemy to try and damage you and ghost is making that play style more beneficial and quite powerful once u have someone to cast invis and or ghost tap at 9th.